Critical porosity and elastic properties of microporous mixed carbonate-siliciclastic rocks

Abstract

By integrating elastic-property measurements and quantitative mineralogic and petrographic analyses of 45 mixed carbonate-siliciclastic samples from two wells drilled in Late Cretaceous rock of the South Provence Basin (southeast France), we can (1) identify and quantify the parameters controlling elastic properties; (2) demonstrate that micrite can be considered as a porous medium with a low critical porosity, averaging 18%; and (3) relate diagenetic transformations, pore-structure modifications, and elastic-property changes. Microporous carbonates with compact anhedral and euhedral microrhombic micrites display a steeper decrease in compressional and shear velocities with increasing porosity than do carbonate rocks with moldic, intergranular, or intercrystalline macroporosity. The low value of critical porosity estimated in micrites (18%), as well as the steep slopes of velocity-porosity relationships at low porosity, is believed to result from a pore-network geometry characterized by very flat, thin pores bounded by planar faces of micrite crystals. Cementation of microrhombic micrite steeply increases elastic moduli, whereas dissolution processes significantly increase porosity with low variations of elastic moduli. Thus, critical-porosity concepts can help describe and model elastic properties of micritic microporous carbonate.